Selenium and/or inorganic and organic selenium compounds are valuable catalysts or co-catalysts in the production of urethanes, for example from organic nitro compounds, carbon monoxide and organic hydroxyl compounds (cf., for example, U.S. Pat. Nos. 3,895,054 and 3,956,360; British Pat. No. 1,485,108 and 1,486,399; German Patent Applications P 28 08 980.7, P 28 08 990.9 and P 28 38 754.4).
The urethanes produced during these synthesis processes often contain traces of selenium which may be present in the form of elementary selenium or in the form of inorganic or organic selenium compounds. In particular, the combination of selenium with organic compounds in the above-mentioned processes often lead to the formation of organic selenium compounds or other soluble selenium compounds which remain in solution together with the urethane formed during the reaction and consequently contaminate the urethane during removal of the organic hydroxyl compounds commonly used as solvents.
Due to the toxicity and the cost of selenium, it is necessary to separate the selenium materials from the urethanes formed. In doing this, it is necessary to isolate the separated selenium compounds in a form which is suitable for re-use as a catalyst.
There has, therefore, been many attempts to provide processes for purifying selenium-containing urethanes. For example, U.S. Pat. Nos. 4,038,375; 4,038,376; 4,038,377; 4,041,139; 4,055,629; and 4,055,630 wherein purification has been carried out by means of liquid-liquid extraction or by means of metal oxides and metal salts such as mercury compounds. The selenium is generally only partially removed by these processes, even when adopting complicated modes of operation and using toxicologically critical substances. Progress has been made by the process described in U.S. Pat. No. 4,130,633 wherein aqueous hydrogen peroxide solutions are used to remove the selenium or selenium compounds. In this method it is unnecessary to use toxicologically critical substances such as mercury compounds. The process according to U.S. Pat. No. 4,130,633 is, however, also accompanied by serious drawbacks because the urethanes produced in this way are unsuitable for commercial use due to their high residual content of selenium and because the additional step of separating any hydrogen peroxide remaining in the urethane solution is a process which is itself not free from difficulties.
The object of the present invention was therefore to provide an improved process for purifying selenium-containing urethanes in which the selenium content in the polyurethane can be drastically reduced in a simple manner and in which the formation of hydrogen peroxide-containing urethanes or urethane solutions is avoided.
This object could surprisingly be achieved by means of the process according to the present invention wherein the above-mentioned drawbacks are substantially overcome.
It has also surprisingly been found that the method according to the present invention is also eminently suitable for removing sulfur or sulfur compounds from urethanes.